4-digit numbers ending with 1

I'm trying to find something interesting to discuss about such numbers:

1101
1201 = 24^2 + 25^2 ........... 1201^2 = 49^2 + 1200^2
1301 = 25^2 + 26^2 ........... 1301^2 = 51^2 + 1300^2
1401
1501
1601
1701
1801
1901
2001
2101
2201
2301
2401
2501
2601
2701
2801
2901

etc

3-digit numbers

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811

821

831

841

851

861

871

881 = 16^2 + 25^2 .............. 881^2 = 369^2 + 800^2

891

901

911

921

931

941

951